The use of silicon carbide for treating cast iron melts, such as for siliconizing, carburizing, deoxidizing and inoculating, has long formed part of the prior art (see U.S. Pat. No. 2,020,171, DE-C-2215266 and DE-A-2746478).
For this purpose there is customarily used so-called metallurgical silicon carbide which has a SiC content varying within the range of approximately 85-95% by weight, which still contains, as a result of the manufacturing process, approximately 2-5% by weight of free carbon and approximately 2-3% by weight of silica and which is commercially available in the form of granules within the range of up to 20 mm and having a maximum grain distribution of &lt;10 mm.
The use of metallurgical silicon carbide as an alloying agent has a positive influence on the quality of the cast iron since, during the siliconizing step, a preliminary inoculation of the melt occurs and the effect of this preliminary inoculation disappears only slowly. It manifests itself in a slight super-cooling of the melt, an increase in the number of eutectic granules, a favorable distribution and formation of the graphite and a reduced tendency to white hardening and an increased tendency to grey hardening. This results in an increase of the tensile strength:hardness relationship producing better mechanical properties, machineability and general homogenity throughout the casting (see investigations of K. H. Caspers in "Giesserei" (Foundry), Vol. 59 (1972), pp 556-559 and the summarizing article to Th. Benecke in "Giesserei" (Foundry), Vol. 68 (1981), pp 344-349).
Virtually nothing is known about the causes of the inoculating effect of metallurgical silicon carbide and its long-term effect. From the comparative studies of R. L. Doelmann, et al in "Giesserei-Praxis" (Foundry Practice), No. 12 (1981), pp 205-212, it seems that with 80% silicon carbide, a cast iron can be produced having better properties than that produced with 90% silicon carbide, which the authors attempt to associate with the higher carbon content (7.2%) in the form of graphite and thermally treated petroleum coke in the 80% silicon carbide.
This does not, however, permit the specific selection of particular types of metallurgical SiC which produce, each time, in a reproducible manner, the same results under the same smelting conditions, since the individual factors which are responsible, when SiC is used, for the formation of nuclei in the melt are still unknown.
The problem is, therefore, to select a silicon carbide for the treatment of cast iron melts that is of such a type that it is able specifically to control the formation of nuclei in the cast iron melt, without it being necessary, for this purpose, to carry out an expensive preliminary assessment of commercial metallurgical SiC qualities, each of which, as a result of the manufacturing process, contains different amounts of accompanying materials.